1. Field of the Invention
The present invention relates to water decontamination, and more particularly, to apparatuses and processes for removing organic contaminants by OH.sup.- oxidization in water by mixing ozone and hydrogen peroxide into contaminated water.
2. Description of the Related Art
Heightened awareness of the risks to human health posed by environmental contaminants has led to imposition of stringent regulation on levels of contamination in drinking water. For example, the current maximum concentration of trichloroethylene (TCE) permitted by the United States Environmental Protection Agency is 5 ppb. TCE belongs to a class of compounds known as volatile organic contaminants, or VOC's. Because of their toxicity and/or carcinogenic properties, VOC's must be removed before water can be utilized for most purposes.
Controlled oxidation of hazardous organic contamination in contaminated water is increasingly accepted for decontamination. One example is the so-called "advanced oxidation process" (AOP), wherein ozone (O.sub.3) and hydrogen peroxide (H.sub.2 O.sub.2) introduced into water react with each other to form the hydroxyl radical (HO.multidot.), a powerful oxidizing species. Hydrogen peroxide, ozone, and hydroxyl radical then come into contact with and oxidize contaminants, destroying them. Glaze and Kang, J. Amer. Water Works Assoc., 80, 51 (1988) describe an advanced oxidation process wherein ozone (O.sub.3) and hydrogen peroxide (H.sub.2 O.sub.2) are introduced into contaminated water at atmospheric pressure.
Known AOP decontamination systems suffer from a number of serious disadvantages. First, the rate of ozone destruction in conventional systems has been documented as being initially very rapid. However, no corresponding rapid destruction of contaminants during the initial mixing of ozone and hydrogen peroxide in water has been observed or reported. Thus, conventional oxidation decontamination processes utilizing ozone are relatively inefficient, consuming large quantities of relatively expensive ozone while eliminating only modest amounts of contaminants.
Therefore, it is desirable to design oxidation decontamination processes and apparatuses utilizing ozone that enhance mixing, and hence reduce the time required for ozone, hydrogen peroxide, and/or the hydroxyl radical to encounter contaminants present in the water, thereby maximizing oxidation.
A second disadvantage of known AOP decontamination systems is formation of unwanted disinfection byproducts. For example, bromide ions (Br.sup.-), naturally present in the water, can undergo a series of reactions to produce bromate (BrO.sub.3.sup.-): EQU 3Br.sup.- +O.sub.3 (only).fwdarw.3BrO.sup.- (1) EQU BrO.sup.- +(O.sub.3 or HO.multidot.).fwdarw.BrO.sub.3.sup.-(2)
Bromate has recently been designated as a suspected carcinogen, and the U.S.E.P.A. has established a maximum level for drinking water of 10 .mu.g/L. It is thus important to prevent or minimize bromate formation during decontamination of potable water.
In step (1) above, neither the hydroxyl radical (HO.multidot.) nor hydrogen peroxide alone oxidize bromide to form hypobromite (BrO.sup.-). Moreover, reaction (2) must compete with the conversion of hypobromite back to bromide that occurs in the presence of hydrogen peroxide: EQU BrO.sup.- +H.sub.2 O.sub.2 .fwdarw.Br.sup.- (3)
Thus when hydrogen peroxide concentration is greater, reaction (3) is favored and the formation of bromate is discouraged.
Therefore, it is desirable to develop decontamination processes and apparatuses utilizing ozone and hydrogen peroxide wherein residual ozone concentrations are minimized and hydrogen peroxide concentrations are maximized in order to suppress the formation of bromate.
A third disadvantage of conventional ozone decontamination systems is the limited solubility of ozone in water at atmospheric pressure. FIG. 1 shows that the solubility of ozone in water increases with higher pressure. However, conventional oxidation decontamination systems introduce ozone at only atmospheric pressure, limiting the amount of ozone that can be dissolved in the water.
Therefore, it is desirable to design decontamination processes and apparatuses wherein ozone is introduced and maintained in solution within the contaminated water under greater than atmospheric pressure. As a result, more ozone is dissolved in the water and available to react with hydrogen peroxide to form OH.sup.- and oxidizable contaminants.
A fourth disadvantage is the limited concentration of ozone normally present in the reactant gas stream that is mixed with the water. FIG. 2 shows that ozone solubility in water increases with increasing ozone in the gas phase. Conventional oxidation systems utilize gas streams containing only about 1-4% ozone by weight in air, effectively limiting the amount of ozone soluble in water.
An additional problem associated with the introduction of ozone in a stream of air is that the air can strip the water of VOC's and ozone, hindering the oxidation process and creating a waste gas stream that must be separately decontaminated.
Therefore, it is desirable to design oxidation decontamination processes and apparatuses wherein ozone is generated from oxygen, constituting a larger percentage of the reactant gas introduced into the water. This results in more ozone being dissolved in the water and preventing stripping of ozone and VOC's.
A fifth disadvantage is that the ozone and hydrogen peroxide are generally introduced into a side stream of contaminated water that has been diverted from the main flow in order to receive the ozone and the hydrogen peroxide. The resulting elevated concentrations of ozone in the side stream relative to the entire flow creates several problems. First, subsequent introduction of the side stream may result in uneven mixing of the ozone in the overall water flow. Second, introduction of the ozone within the smaller volume of the side stream necessarily increases the local concentration of ozone and may lead to increased bromate formation.
Therefore, it is desirable to design decontamination processes and apparatuses wherein ozone and hydrogen peroxide are injected "in-line" with the entire contaminated water flow to achieve uniform and rapid mixing of ozone, and minimize local concentrations of ozone.